1. Field of the Invention
The present invention relates to an organic semiconductor, i.e. a two-terminal element diode which exhibits a nonlinear current-voltage characteristics by the movement of electrons and holes supplied to the junction of thin films made of organic compounds having electrical characteristics different from each other.
2. Description of the Related Art
An organic electroluminescence element (hereinafter also referred to as xe2x80x9corganic EL elementxe2x80x9d) is known which makes use of electroluminescence (hereinafter also referred to as xe2x80x9cELxe2x80x9d) of an organic compound that emits light by injection of electric current and includes a light-emitting layer formed by a thin layer of such an organic compound. Now, an organic EL element display device formed by arranging a plurality of the above organic EL elements in a matrix form is receiving attention as a display device capable of achieving low power consumption, high display quality, and thickness reduction. As shown in FIG. 1, each organic EL element 200 is comprised of a transparent substrate 1, such as a glass plate or the like, a transparent anode electrode 201 formed on the substrate, at least one organic material layer 202 which is comprised of an electron transport layer, a light-emitting layer, and a hole transport layer, and a metal cathode electrode 203, all of which are laminated in the mentioned order. When a positive voltage is applied to the transparent anode electrode 201, and a negative voltage is applied to the metal cathode electrode 203, that is, when a direct current is caused to flow between the transparent electrode and the metal electrode, the light-emitting layer in the organic material layer 202 emits light.
The organic EL element is considered as a capacitive light-emitting element. When a direct current driving voltage for causing light emission is applied between the electrodes, an electric charge is accumulated in a capacity component. Then, when the driving voltage exceeds a light-emitting threshold voltage of the EL element, a current starts to flow in the organic material layer to cause the organic EL element to emit light at a luminous intensity approximately proportional to the current.
The organic EL element display device is a light-emitting apparatus composed of a plurality of light-emitting pixels, that is, organic EL elements, in an image display layout in which the organic EL elements are disposed at respective points of intersection of horizontal lines and vertical lines, i.e. in a so-called matrix form. An example of the method of driving the organic EL element display device is a so-called simple matrix driving method. A display apparatus based on the simple matrix driving method has a plurality of anode lines and cathode lines arranged in the form of a matrix (grid), with organic EL elements connected at respective points of the intersection of the anode lines and the cathode lines. Either of the anode lines and the cathode lines are sequentially selected at predetermined time intervals for scanning, and the other lines are driven by a drive source in synchronism with the scanning, to cause organic EL elements arranged at desired points of intersection of the lines to emit light. In this method, each organic EL element is lighted only during an access time period, and hence to increase the size of a display screen of the apparatus, a large current and a high voltage are required.
To increase the size of the display screen of the organic EL element display device, an active matrix driving method may be considered, besides the simple matrix driving method. The active matrix driving method uses a thin film transistor (TFT) arranged at each point of the intersection (of the anode lines and the cathode lines), and supplies a current to each pixel by switching operation to cause the associated organic EL element to emit light. The TFT can be formed with using an element formed by p-Si or a-Si. Further, it is also possible to form the TFT by using a MOS-FET (Metal Oxide Semiconductor Field Effect Transistor) in place of the above element. Accordingly, the active matrix driving method requires a large number of switching circuit substrates, and inorganic materials have to be laminated on substrates of the circuits. Therefore, a high temperature process is employed for manufacturing the switching circuits.
To reduce the instantaneous brightness of light-emitting elements to prevent deterioration or breakdown of both of the driving methods above mentioned, it is required to apply reverse voltage to them when light is not emitted, i.e. the scanning is inhibited. Therefore, it is proposed to provide a rectifier connected in series to the light-emitting element.
Therefore, an object of the present invention is to provide an organic semiconductor diode which can be produced at a relatively low temperature, and another object of the present invention is to provide an organic EL element display device having the organic semiconductor diodes formed on a common substrate.
An organic semiconductor diode according to the present invention comprises: a hole transport layer which is arranged on an anode side and formed of an organic compound having a hole transport capability; and an electron transport layer which is arranged on a cathode side and formed of an organic compound having an electron transport capability, the hole transport layer and the electron transport layer being laminated one upon another, wherein the cathode has a work function close to or smaller than the electron affinity of the electron transport layer, while the anode has a work function larger than that of the cathode, whereby the organic semiconductor diode exhibits nonlinear current-voltage characteristics when a voltage is applied between the hole transport layer and the electron transport layer in contact with each other.
In another aspect of the present invention of the organic semiconductor diode, said hole transport layer is formed of a porphyrin derivative.
In a further aspect of the present invention of the organic semiconductor diode, said electron transport layer is formed of a perylene derivative.